Hitherto, a loop network system has been widely used for the data communication between two or more computers. The loop network system of this type is generally simple in arrangement, although the data circulating around the network creates an undesired synchronization lag or a great retardation due to synchronization lag correction.
Further, it has been well known to provide a hairpin network in which a loop shape network is provided at an owner station which monitors and controls the transmission operation of the network. The hairpin network of this type generally comprises an owner station OS from which the hairpin network NW extends with two or more member stations MS.sub.1 to MS.sub.n connected thereto in a given order and having the transmission terminals T.sub.1 to T.sub.n and the reception terminals R.sub.1 to R.sub.n respectively connected to a transmission line T.sub.L and a reception line R.sub.L of the hairpin network NW as shown in FIG. 1.
In the network system as described in the foregoing, when data communication between the member stations MS.sub.1 and MS.sub.n is to take place, a vacant time slot transmitted from the owner station to the network NW is detected and reserved by a member station MS.sub.1. Data is provided by the member station MS.sub.1 through the transmission line T.sub.L of the network NW and the data is received by another intended member station for example MS.sub.n by way of the reception line R.sub.L. Accordingly, in this system data communication between the member stations is enabled through the owner station OS without causing any undesired problem as is normally seen in other conventional loop network system.
A general data format of the time slot transmitted from the owner station is illustrated in FIG. 2 where a frame of an individual time slot includes an initial flag, an address, a command, a data, an error checking code and an end flag in the given order. The time slot is reserved and written in the initial flag, while the specific member station to which data is to be transmitted is written in the address and the specific data to be transmitted is written in the command.
The owner station OS for transmitting the time slot is generally composed of a clock signal generator and a matching terminal as shown in FIG. 3. The time signal derived from the clock signal generator is transmitted to the slot signal generator to provide a time slot which is in turn transmitted to the transmission line T.sub.L. The reception line R.sub.L terminates at the matching terminal. The member station MS for effectuating the transmission as well as the reception with the time slot is generally arranged as shown in FIG. 4. The usual member station MS comprises a computer or a terminal equipment thereof and a buffer through which the data to the transmission line T.sub.L is transmitted by way of a bus detector, a transmission controller and a transmission changer. Data is transmitted to the reception line R.sub.L through the buffer as a function of the reception data as well as an error detector and a reception controller. Further, the bus detector the synchronization with the time slot and detects the vacant time slot is detected.
In the hairpin network of the foregoing type, when data communication between member stations is to take place, a member station must access a vacant transmitted time slot for reservation. A problem arises if access of the vacant time slot is desired by a member station MS.sub.1 nearer to the owner station OS than a succeeding member station MS.sub.n. The reason is that the writing time for accessing vacant time slot for a succeeding, more distant, member station is increased so that access by the nearer member station is enhanced.
To overcome the disadvantages of the conventional hairpin network, it has been proposed to provide a system ensuring equal access of the vacant time slot for both near and the more distant member stations by reserving the vacant time slot at the transmission terminal and the reception terminal of the member station and by limiting access to vacant time slots at the transmission terminal and at the reception terminal as for example, described in Japanese patent publication No. 54-114903. Namely, according to this conventional allotting system of the vacant time slot the transmission terminals T.sub.1 -T.sub.n and the reception terminal R.sub.1 -R.sub.2 have time slot monitoring functions in individual member stations. For example, in the transmission terminal the vacant time slot frame has a heading bit in the frame of an uneven order which is zero while in the reception terminal the vacant time slot frame has a heading bit in the frame of an even order which is also zero. Accordingly, when a transmission member station has traced a desired vacant time slot, the heating bit of the traced vacant time slot is changed to "1" for reservation, thereby, inhibiting utilization by the other member stations or by the owner station. Thus, when the transmission terminal has located a vacant time slot, the heading bit of the vacant time slot becomes the number "1" enabling simultaneous writing of the identity of the transmission station and that of the reception station as well as the data to be transmitted in the given order. When the vacant time slot has been located at the reception terminal, the heading bit of the vacant time slot is also changed to number "1" which inhibits utilization by the other member stations or the owner station. On the other hand, the reception member station monitors the time slot at the reception terminal and when the time slot for the reception member station has been traced, the transmission terminal of the transmission member station is connected to the reception member terminal of the reception member station through the transmission line for reception of the data from the transmission member station. In the reception member station, when the predetermined data transmission from the transmission member station is completed, the heading bit of the reserved time slot is changed to "0" thus providing a vacant slot which is freed for utilization by the other member stations, the owner station and the like.
However, in the system for allotting a vacant time slot hereinbefore described, the control system for reservation by way the heading bit of the time slot is complicated, lacks reliability for detecting the trouble on transmission line and suffers from other difficulties arising from the single network system for data communication.
The foregoing disadvantages and shortcomings of the conventional hairpin network system have been overcome in accordance with the invention by providing an improved computer network system in which two hairpin networks are disposed between two owner stations, i.e. a station which transmits a vacant time slot, with two or more member stations, i.e. a station for carrying out material data communication arranged in a given order from one owner station to the opposite owner station along the two networks and each member station is provided with two sets of transmission and reception terminals of which one set is connected to the transmission line and reception line of one network and the other set is connected to the transmission line and the reception line of the second network. Namely, in accordance with the invention, each member station is connected to one network with one set of transmission and receiver terminals in the precedent position to one owner station, while the same member station is connected to the other network with its other set of transmission and receiver terminals in a reverse or subordinate position to the other owner station. Hence chances of access of the vacant time slot transmitted independently from the owner station of each network by the member stations are balanced.